This invention relates to the control of viscosity. In one application, the invention is concerned with controlling the viscosity of clay suspensions created by mineral processing operations.
In the mineral processing industry, problems are encountered if the viscosity of slimes, which are to be disposed of in a slimes dam or other disposal facility, is too high. Amongst such problems are those of low pumpability and high wear of mechanical equipment.
A clay particle, when introduced to an aqueous medium, may assume a surface charge due to interaction with this medium. The magnitude of this charge is dependant on the surface chemistry of the solid and the composition of the medium. Where the concentration of charged particles in the medium is high, adjacent particles may be sufficiently close to one another allow the predomination of electrostatic interactions which are greater in magnitude than the gravitational force exerted on individual particles.
This results in a colloidally-stable suspension in which, by definition, the contained particles can not migrate downwardly under gravity.
In order to effect solid/liquid separation by gravitational means, a common approach is to reduce the surface charge on the particles either by modifying the ionic composition of the medium or by chemical reaction between an introduced species and the particle.
The charge on the particle surface can be approximated by measurement of a bulk property known as zeta-potential. The zeta-potential is defined as the electrical potential which exists between the particle surface and the layer of counter-ions which surround it in the aqueous medium. Particles having a zeta-potential of -15 mV&lt;.xi.&lt;15 mV are observed to settle naturally in aqueous media. Under these conditions, London--Van der Waal's forces predominate over the repulsive forces and mutual attraction takes place between adjacent particles. Discrete agglomerates of coagulated particles, termed "flocs", are thereafter able to settle at a greater velocity due to their increased effective mass.
The particle orientation contained within colloidal clay suspensions is the result of the electrochemical and electrostatic interactions between the particle surface and the medium. The bulk rheology is therefore indicative of this orientation and it is implicit that the rheology may be altered by modifying the composition of the medium.
Where the concentration of particles in suspension is sufficiently high, reduction of their zeta-potential can lead to spontaneous gelation through mutual interaction of the particles. In some suspensions, zeta-potential can be reduced by increasing the pH of the mixture and in others by adding an inorganic salt.
Gelation is undesirable in mineral processing applications since the resultant highly viscous gel may be extremely difficult to dispose of using conventional slimes disposal techniques.
The present invention seeks to provide a process for treating a colloidally-stable solid/liquid mixture so that undesirably high viscosity levels are avoided.